Summary:
The anaerobic nucleoside-triphosphate reductase activating system is composed of three enzymes and several compounds. Anaerobic nucleoside-triphosphate reductase is activated through the action of a specific activating enzyme, nucleoside-triphosphate reductase activase, flavodoxin NADP+ reductase, S-adenosylmethionine, flavodoxin and NADPH. All of these components form a multi-enzyme complex with the ribonucleoside reductase itself. [Bianchi93, Sun95a]

Summary:
The NrdD reductase is activated by the NrdG activase under anaerobic conditions and is inactivated by oxygen. The protein is highly sensitive to O2.

An nrdD null mutant does not grow under entirely anaerobic conditions, but grows under aerobic or microaerophilic conditions due to the activity of NrdA and/or NrdB [Garriga96].

Ribonucleotide reductase catalyzes the rate-limiting step in DNA biosynthesis. Its central role in DNA replication and repair makes its regulation important to ensure appropriate pools of deoxyribonucleotides for these processes. Three major classes I, II and III have been designated that share similar catalytic mechanisms. Enterobacteria, including Escherichia coli and Salmonella enterica serovar Typhimurium contain class Ia (encoded by nrdA and nrdB), class Ib (encoded by nrdE and nrdF) and class III (encoded by nrdD) enzymes. Class Ia and Ib enzymes are active under aerobic conditions, while class III enzymes are inactivated by oxygen and function under strictly anaerobic conditions. Although there are differences in structure and cofactor use, their catalytic mechanisms involve a transient cysteinyl radical at the active site that inititates ribonucleotide reduction. Regeneration of the enzymes is accomplished by corresponding reductive enzyme systems [Gon06] and discussed in [Torrents07].

This enzyme is a class III ribonucleotide reductase that is essential for anaerobic growth. In E. coli strain DHB4, construction of deletion mutants and analysis of their growth characteristics showed that the main ribonucleotide reductase activity in strict anaerobiosis is provided by the products of genes nrdD and nrdG. Under highly restricted oxygen condtions, the products of genes nrdA and nrdB can support some growth in a strain containing a deletion of nrdD and nrdG. Either thioredoxin reductase or glutathione reductase are required for anaerobic growth, suggesting that these reductive pathways contribute to enzyme activity [Gon06].

Summary:
An nrdG null mutant does not grow under entirely anaerobic conditions, but grows under aerobic or microaerophilic conditions due to the activity of NrdA and/or NrdB [Garriga96].

The NrdG activase activates the NrdD reductase under anaerobic conditions.

Using site-directed mutagenesis, the participation of three cysteine residues in iron chelation in the (4Fe-4S) cluster of this enzyme was demonstrated, but a fourth ligand remained unidentified [Tamarit00].